X-rays have been used in radiographic imaging for a long time, and have been subject to great developments. In its simplest form, imaging is conducted by providing a source of X-ray radiation, an object to be imaged, through which the radiation is transmitted, and a detector for the detection and recording of the transmitted radiation. The X-ray detector used today, at hospitals, is normally a screen-film combination. In a phosphor screen (e.g. Gd.sub.2 O.sub.2 S), X-ray photons are converted and thereby produce secondary light, which is registered on a photographic film. The use of a film limits the dynamic range of the image. The increased efficiency achieved by using a phosphor screen is provided at the expense of the resolution, since the secondary light is emitted isotropically.
To visualize an object within an image, it is necessary that the signal to noise ratio exceeds a certain threshold. The ideal system would have the image noise determined only by photon statistics. This is typically not the case for systems operating with a screen-film combination. To obtain a useful diagnostic image one has hence to increase the patient dose of X-ray radiation. X-ray photon flux is, by nature, digital. However, one has to distinguish between two different methods in producing digital images:
Integrating technique is an intrinsically analogue method. The response in each pixel is proportional to the total X-ray energy flux. The image is then built up digitally by means of the pixels. Examples of the integrating approach to imaging are CCD (charge-coupled device), storage phosphors, selenium plates, etc. The dynamic range of many of these "digital" detectors is similar to that of film. As in the film technique, the photon flux energy (not the number of photons) is integrated, and thus add noise, since X-ray tubes produce a wide energy spectrum. The most significant noise sources are the "dark current" and the fluctuations in photon energy. PA1 Photon counting is an intrinsically digital method, in which each photon is detected, and detection signals are counted.
A two-dimensional photon counting detector requires many readout elements, and a huge number of interconnections will be needed. This leads to typical manufacturing and reliability problems, which has been experienced in such systems. It would be difficult to make a large two-dimensional detector with high resolution and high probability for interaction of a major fraction of the X-ray photons.
Another drawback of two-dimensional detector readout systems relates to the fact that the X-ray flux coming from the X-ray source is divergent. In a thick conversion volume of the detectors this divergence causes a parallax error. Most methods proposed to minimize the parallax error are difficult to implement in practice.
One way to overcome size and cost limitations, connected to two-dimensional detector readout systems, is to create an image receptor that is essentially one-dimensional and acquires the second dimension for the image by scanning the X-ray beam and detector across the object to be imaged. Scanning can be done by employing a single line detector and a highly collimated planar X-ray beam. In addition, this approach eliminates the scattered radiation noise but imposes a large heat load on the X-ray tube. To ease the tube loading and simplify the mechanics (by reducing the scanning distance), a multiline set of low cost one-dimensional detectors is beneficial.
One advantage with a line detector is a significant reduction of image noise, which is caused by radiation scattering in the object to be imaged. An X-ray photon that is Compton-scattered in the object will not be detected in a line detector.
Several attempts have been made to develop a photon counting X-ray imaging system based on the scanning technique. This requires detectors that produce fast signals with a rise time of a few nanoseconds. Only a few detection media can produce signals that fast, e.g. a gas or a semiconductor (for example silicon). Semiconductor detectors are expensive and are thus not practical in a multiline configuration. In a gas medium, an X-ray photon interacts with a gas atom which emits a primary ionization electron, which in its turn produces electron-ion pairs that are further multiplied in a gas avalanche. The advantage of a gas detector is low cost, a high noiseless signal amplification in the gas (up to 10.sup.6), and a uniformity of the detection media.
Several imaging systems described in published articles utilize a multiwire proportional chamber as detector. In its basic configuration, the multiwire proportional chamber consists of a set of thin anode wires stretched between, and parallel with, two cathode planes. Application of a voltage between the anode wires and the cathode planes creates an electric field in the chamber. Electrons emitted in the gas by ionization of gas atoms, caused by incident X-ray photons, drift towards the anode wires, and when approaching the thin wires they experience ionizing interactions, with gas molecules, in the strong electric field. The ensuing avalanche multiplication provides a noiseless amplification of the charge signal, by a factor as large as 10.sup.5 or more.
An example of a digital imaging system based on photon counting is described in the article, "Multiwire proportional chamber for a digital radiographic installation", by S. E. Baru et. al., in Nuclear Instruments and Methods in Physics Research A, vol. 283 (Nov. 10, 1989), pages 431-435. This detector is a combination of a drift chamber and a multiwire proportional chamber with non-parallel anode wires aiming at the focal point of the X-ray source. The radial wires enable the use of a thick interaction volume without parallax error. The uniformity of gain along the anode wires is guaranteed by an increasing gap between the anode wires and the cathode planes.
The described device has, however, the following drawbacks.
The need for providing sufficient space for wire mounting and high voltage isolation results in losses of X-ray detection efficiency.
The use of radial wires to solve the parallax problem results in a position resolution limited by the smallest practical anode wire pitch of about 1 mm. The problem can be overcome by using cathode strip readout that provides the ultimate multiwire proportional chamber resolution. One possibility of a practically feasible fast cathode strip readout is described in the article, "The OD-3 fast one-coordinate X-ray detector", by V. M. Aulchenco et. al., in Nuclear Instruments and Methods in Physics Research A, vol. 367 (Dec. 11, 1995), pages 79-82. In this solution, an increasing anode-cathode gap is combined with a decreasing high voltage applied to different anode wire groups.
A known problem with using multiwire proportional chambers for medical imaging is the space charge effect that degrades the detector performance at high X-ray fluxes above 10 kHz/mm.sup.2. To decrease the space charge effect, the anode plane has been modified by adding alternating cathode wires in a prior art device, disclosed in U.S. Pat. No. 5,521,956 (G. Charpak).
The use of thin wires (typically less than 100 .mu.m in diameter) in multiwire proportional chambers makes them difficult to construct, and reduces reliability, since one broken wire disables operation of the whole detector.
A gas avalanche detector that is very simple in construction and does not use anode wires is the gaseous parallel plate avalanche chamber. This detector is basically a gas-filled capacitor, comprising two parallel conducting plates, an anode and a cathode, subjected to a high voltage. The high voltage is chosen such that electrons released by ionization in the gas produce avalanches in a strong electric field between the plates. Typically, the distance between the plates is of the order of one millimeter, and the field strength is in the order of kilovolts per millimeter, depending on the type of gas used. A wide variety of gases can be used depending on the application. In such a detector X-ray photons are incident on a plane parallel to the detector plane, or on the cathode, which is made of a material that emits electrons, so called photoelectrons, when X-ray photons interact with it.
An important advantage over the multiwire proportional chamber, is that the electrostatic field in a gaseous parallel plate avalanche chamber is not concentrated around single thin wires, but is constant over the entire amplification volume. This results in a very short drift time of positive ions across the amplification gap, thus drastically reducing the (space charge effect.
Another advantage of a gaseous parallel plate avalanche chamber is that the surface area of the anode is much larger than that of a multiwire proportional chamber (the anode wires). Thus the detector aging due to depositions on the anode is much smaller.
A further advantage of a gaseous parallel plate avalanche chamber is that the fast electron signal represents a considerable fraction of the total induced charge. It is about 10% of the total signal at gains around 10.sup.5, as compared to 1% in multiwire proportional chambers.
A still further advantage of a gaseous parallel plate avalanche chamber is the simple shape of signals induced on electrodes by the movement of avalanche ions. Thus, the signal processing electronics does not require an ion tail cancellation stage, as needed in high speed readout of a multiwire proportional chamber. Since the ions in a gaseous parallel plate avalanche chamber move in a uniform field with constant velocity a simple differentiation removes their contribution, leaving a very fast electron signal.
An example of using a gaseous parallel plate avalanche chamber for radiographic imaging is described in the article, "A parallel plate chamber with pixel readout for very high data rate", by F. Angelini et. al., in IEEE Transactions on Nuclear Science, vol. 36 (February 1989) pages 213-217. In the two-dimensional readout configuration described, it is impossible to achieve high X-ray conversion efficiency despite the addition of a drift chamber in front of a parallel plate chamber to increase the thickness of the gas layer.
Another device, disclosed in U.S. Pat. No. 5,308,987 (Wuest et. al.), utilizes a cathode made of a high atomic number material to improve the conversion efficiency in a parallel plate chamber used in a two-dimensional readout configuration. The low yield of photoelectrons from the high atomic number material results in a reduction of X-ray ray detection efficiency.
Another important difference from a multiwire proportional chamber is that the gas amplification factor strongly depends on the distance from the primary ionization charge to the anode, resulting in a poor energy resolution and signal detection efficiency, in prior used gaseous parallel plate avalanche chambers. Due to this problem, prior devices were unable to use the gas amplification gap in gaseous parallel plate avalanche chambers as an X-ray conversion volume. This limitation is overcome in this invention by providing a well collimated planar beam incident sideways on the detector.
In addition to the advantages described above, the use of a thin planar X-ray beam simplifies the construction of the detector entrance window, since it is easier to contain a gas pressure with a slit window than over a large area. The use of a thin foil minimizes losses of X-ray photons in the detector entrance window.